This straight forward circuit will protect electrical appliances from over voltage as well as under voltage. The circuit also produces an alarm when the power supply comes back. An ideal circuit for home to protect your valuable equipments from voltage fluctuations. The same circuit with some modifications can be used  to make a automatic voltage stabilizer.

When the mains voltage is in the normal level, the voltage at the negative terminal of zener diode D4 will be less than 5.6 Volts. At this condition transistor T1 will not conduct. The same time voltage at the negative terminal of zener diode D5 will be greater than 5.6 and so the transistor T2 will be conducting. The relay will be activated and the green LED will be glowing.

When the mains voltage is higher than the set limit the transistor T1 becomes conducting since the voltage at the negative terminal of  D4 is greater than 5.6 V. At the same time transistor T2 will be non conducting which results in the deactivation of relay to cut the mains supply from load. When the mains voltage is less than the set limit transistors T1 & T2 becomes non conducting  making the relay to de-activate and cut the load from mains.

The timer NE555 is wired as a monostable multivibrator with a pulse width of 10ms.When the power comes back after a cut off a negative voltage is obtained at the trigger pin which triggers the IC NE555. The transistor T3 gets forward biased and it drives the buzzer to produce a beep as an indication of power resumption. Also the transistor T1 is made on which in turn makes T2 off. As a result the relay will remain de- activate for 10ms and this provides the sufficient delay and the equipment  is protected from surge voltages.

Circuit diagram with Parts list.Click to enlarge.



  • To calibrate the circuit a autotransformer is needed. Connect the output of autotransformer to the transformer primary.
  • Set the voltage to 260V and adjust  VR1 to make the relay deactivated.
  • Now set the autotransformer to 160V and adjust VR2  so that the relay is de-energized.
  • VR3 can be used to vary the volume of buzzer.


  1. Tapan Kumar Das

    Where is VR2 in the ckt to adjust low voltage. Please check.

    T K Das

  2. Priyanka verma

    sir plss rply fast bcz the use of autotransformer are importan can i use varic instead of auto transformer

    • neeraj kumar

      can u plz send me the project report of this project,its urgent for me.

  3. priyanka verma

    pls help us bcz we cant understand working of this we made this project but we cant understand how it operate

    • seetharaman

      eliminate alarm portion to reduce cost further.

  4. isaac yawson

    and also ,i want to know the exact position where it will be connected to ??

    • seetharaman

      It should be connected to input supply circuit. Connect this unit to incoming supply and out put to equipment which requires protection

  5. isaac yawson

    please, i want to know where we will connect the appliance to ouput of the project above ???

  6. g.krishna

    sir,please send me about automatic over load protection system using micocontroller 8086 in embeded system

  7. Adamu Abubakar Muhammad

    I picked High and Low Voltage Cut-Off with Delay And Alarm as my mini project and found this project very interesting because, I succeeded in Implementing the Project without any difficulties. I used a 12-0-12V transformer and BC547 transistors instead of S100. Thanks a lot for your post. Wish to get an explanation on that Car Parking Guard Using Infrared Sensor because I tried it but haven’t get it right due to absence of LM567 IC here. Is there any subtitude to that. What about S567? Thanks a lot.

    • patrick osei

      pls i want to were the output of the circuit is precisely were the appliance is supposed to be connected & also a simplified circuit diagram u used

  8. Tneb kathirasan

    no need to use auto transformer as the component to be assembled in. it is to be tested after finishing this project.Hence one Auto trans needed.You all gets confused without reading the text completely

  9. ajaykumar

    Sir can u plz tell me the functioning ic ne555 timer in the circuit how its getting triggered snd how vwe are using it to make the buzzer on in detail………

  10. Sir,the supply was not going to can i solve this

  11. Keerthi reddy

    Sir,iam ece 2nd year student.i tried this as my mini project.iam working on this since one month.actually the indications leds and buzzer are not responding at crct time.any body had succeeded by doing this project.give me reply

  12. Is there any need to give outside supply to load

  13. Sir,please can u show the connected pcb of this experiment by video or photo

  14. eestudent

    Can anyone tell me where can i connect the appliances please?

  15. ajay kumar

    could u plz tel me what is actually autotransformer is? is ther any other name for it?

    • Seetharaman

      The auto variable transformer is adustable from 0 to 270 volt for an input of 230 volts. It is required to set the circuit accurately, which is also quite expensive to buy, without which adjustment is difficult by trial and error method. the circuit will work with 230 volt ac to the 12-0-12volt transformer primary. the relay NO contact can be used for switching on / off the load.

      • Ajaykumar

        Thanku for ur earlier reply..i am tryng dis project…but d led’s(red nd green) are going at a time when d suppy provided to circuit..plz tell me..a.s.a.p

      • Ajaykumar

        The led’s red and green are glowing at a time when the circuit is given the supply..plz give me d suggestion to make it run..nd plz tell me how to connect load(bulb 60watts) to the relay in which coil terminals we are connecting in d circuit and wat about other 3 terminals..does the load(bulb 60 watts) get d supply from circuit? Are any plz mention…reply a.s.a.p

  16. ajay kumar

    hai i have tried this circuit but i hav not used autotransformer which u have said .i have used 12-0-12v center tapped transformer..but this only provides 12v to the circuit.. how could i supply 230 v supply to make the circuit work… plz reply a.s.a.p

  17. ajaykumar

    Hai….can u plz tel wat is auto transformer since I could not get this in electronics shop. is there any the s name for this

  18. Ajaykumar

    How can we use this circuit to use as a voltage stabilizer plz reply as soon as possible

  19. baqir raza

    Admin…i m.having difficulty in this circuit ,i hv tried to make it as it is shown in schematic the red led is running while the green is off even after changing the voltages..i hv put a double tapped transformer …where to put the auto transformer u r saying..please send me its schematic design which works …i ll be grateful to u

    • Seetharaman

      If red is glowing the power is available for the circuit. Now to adjust the pots for correct operation you have to use an auto transformer (variable voltage transformer to input of the above circuit where it is shown as 230VAC. Adjust variable auto transformer and adjust the pots as shown in the procedure

  20. Hello sir there is show only two terminal connection in figure. But in real there have minimum 4 terminal on relay. So how we can connect remaining terminal on circuit. Please give me location to connect to terminal in figure. Thanks

  21. sarath pa

    i tried this circuit but it is not working properly there is no supply available at the relay terminal so what should i do please replay

    • Seetharaman

      Hi Sarat pa Please use the NO contact of the relay for switching on / off the load. As such no power will be available at relay contacts

  22. Gerwin Dsouza

    if I want to avoid using 555,timer n buzzer i.e. only upto relay1 and t2, then where should input to t1 base from pin 3 of 555 go
    2)can I use 12-0 transformer without center tap and bridge rectifier
    3)is there a pcb designed or using which simple free software can I make a pcb
    very useful circuit thanks a lot,my water pump and tv was burnt due to 440v some time backplease reply to my queries by mail or on this site soon thanks a lot Gerwin Dsouza ,Malvan

  23. I’ve selectd dis as my project… Bt I’m not much familiar wid relay… I found 5 pins on d only 2 terminals of d relay r cnctd in d circuit diagrm… Plz giv me details about d cnctn of relay….

  24. Hi im very interested of your circuit i wish i could build one but my problem is the ic SL-100 it very difficult to it in my country so can you please tell me different thank you looking forward to hear from you.

    • Seetharaman

      HI Electronic buzzer volume cannot be controlled. You can try to paste a thin paper sheet on the sounder hole, this should solve your problem.

  25. saurav rai

    how can we cut off the realay at 260 volt without adjusting VR1??pls help

  26. please, what will i use to set the cut-off voltage since i cant get the autotransformer. And also please what is the power capacity of the circuit? can it be used to power an air conditioner or refrigerator? please i need your reply, thanks and God bless

    • Seetharaman

      Hi Victor for setting an auto transformer is a must.

  27. can you please send me a working schematic diagram?

  28. what circuit can play the same role as the autotransformer,since I could not find it?

  29. can i replace the transformers with the dc supply for 15 v??

    • Seetharaman

      Hi Inten the transformer in the input mains supply sensor for the load, hence it cannot be avoided.

  30. I will be very thankful to u if u reply me fast, please.

    • Seetharaman

      Hi Madhuri when the input voltage is just above minimum value (say 160volts set by VR3) T2 picks up and relay RL1 switches on to give supply to load. If the load increases more than the safe limit (say 260 volts set by VR1)T1 picks up and removes the bias to T2 such that they RL1 switches off cutting off supply to load. once input voltage falls within 190 to 250 volts range the relay RL1 picks up and the supply to load is restored. The timer is to produce beep once the supply is resumed as well to delay the supply to load such that the inlet surge at the time of power resuming is also not sent to the load.

      • If it supplies load even under under-voltage conditions….. Why is it called High/low votage cut-off??… instead we can call it only High voltage cut-off… pls reply fast as we hav chosen this as our project topic….

  31. please give me detailed explination of every component used in this project. please reply me soon

    • sir,
      we select this topic as our project.I Want to know more about the outputs of pleae send me the details of the project as soon as possible
      Thanking you.

    • Seetharaman

      Hi Madhuri there was an error in my earlier reply on voltage level this is correct one. when the input voltage is just above minimum value (say 160volts set by VR3) T2 picks up and relay RL1 switches on to give supply to load. If the load increases more than the safe limit (say 260 volts set by VR1)T1 picks up and removes the bias to T2 such that they RL1 switches off cutting off supply to load. once input voltage falls within 160 to 260 volts range the relay RL1 picks up and the supply to load is restored. The timer is to produce beep once the supply is resumed as well to delay the supply to load such that the inlet surge at the time of power resuming is also not sent to the load.

  32. can you plese sent me the design?and all abt the circuit it working?

  33. Oluwasemiloore

    Pls sir i was given a project to desigh an construct a high an low voltage cut off wit delay an alarm an i dont know how 2go about it pls can u be of help

  34. you are doing a very nice job, i want you to please tell me where my loads are to be connected . and i need more details on how every component works. thanks very much. my email is Hope you reply me a.s.a.p

  35. jyothi singh

    plzz tell me the high-low voltage cutoff with delay and alarm with some modification and advancement in circuit diagram.

  36. plzzzzz tell me the size of the auto transformer to be used, and i didnt get what is no n where exactly the appliance to be checked should be kept ,kind request to you to send me all the details of working of the ckt by tonite


  38. Very nice post my brother but where will the load be connected,and what is the function of auto transformer here….


    From the circuit where do i connect the appliance to be protected for both over/undervoltage conditions.And apart from the auto transformer what else can we use to vary the voltage to the circuit.Also after cutoff how can the appliance be automatically connected to the mains when the fault has been cleared.

  40. Seetharaman

    Hi Balu VR2 is wrongly shown as VR3 in the schematic. For appliance connection please refer to my 6th Feb comment.

  41. Balu prasad

    please help me to find where vr2 is.Also where should we connect the appliance to be protected in case of over voltage?

    • how can we connect appliance operating on 230v to a transformer operating at 12-0-12v secondary? & where to connect the load? please reply….

  42. Seetharaman

    Hi John use the normaly open contact(when the relay is in off condition)of the relay RL1 to switch on off the load.
    Hi ipoy20 this is a working model and a proven design for step voltage regulators.

  43. is this working? please send a reply asap. godbless

  44. seetharaman

    Hi John You can use 2N2219, 2N2218, 8050, 2N1711. any medium power switching transistor can be used.

  45. hey ,thanx for the contribution to technology,i have tried making this project but the sl100 transister is rare so i tried the D313 as an option its still not workin ,pliz suggest another option,thanx in advance.


  46. حسامالورد

    Great and simple circuit, but I searched for circuit that protects the drive three phase 380V AC interruption of one of the stages, but I did not find it, or the opposite one of them.

  47. Can we use transistors instead of integrated circuit IC 555, for the alarm?



  49. I vary d vR1&VR2 but d relay keps suplyin when d voltag is high&low.d crct jst wok lik a stab in dlay.pls hlp

  50. Hello, since i’m working on a circuit for shutting off my diving light when my battery’s get too low, i’d like to integrate this circuit into it. However is it possible to leave everything right of Transistor T2 out of it? And could you please tell me if i could directly use this transistor in series with the base of a BUZ11 MOS-FET, wich I am now using to switch my load. Thanks in advance.

  51. seetharaman

    Hi Hiren Use standard straight or dotted PCB and wireup yourself. For testing you require a variable auto transformer of 0 to 270volt.
    Use the supply to the load to be protected through RLA1 contact

  52. Hi, pls i need details of how the transistors and the Zener diodes work. Pls kindly see me through in my email. Thanks

  53. seetharaman

    Hi Adamu The load should be connected through the potential free contacts of the Relay RLA1 (NO contact- Normally open contact). The relay will switch off/on the load.

  54. Adamu kinami

    U have done a good job. Pls, where do i connect d load n how does d circuit work, detail pls. Give me d write up. My email is Thanks.

  55. i am still awaiting your response in my email, i need you to see me through this. i need details on how the circuit works since i have found this project very interesting and finally approved my supervisor. thanks very much

  56. seetharaman

    Hi Rilwan the NO contact of the ralay RL1 is used for controlling the load. I will be in touch with you soon through your email id

  57. you are doing a very nice job, i want you to please tell me where my loads are to be connected. and i need more details on how every component works. thanks very much. my email is

  58. seetharaman

    SL100 can be substituted with 2N2218, 2N2219 or 2N1711 switching transistors reliably. If you are in India then you can use BEL100N, CL100 also. The above circuit is very reliable one. I have used it in auto phase change over circuit in house. i will be sending the circuit to “Circuits today” soon.

  59. i need someone who has experimented with substitute for sl100 transistor to give me his/her sub.thanks

  60. Hi everybody,I want to know the tripping circuit for high voltage tester circuit.

  61. Plz i want to build this project, with tyhe circuit diagram of automatic voltage stabilizer and the works pls send me the details to my email address.which is thanks.

  62. Plz i want to build this project, with tyhe circuit diagram of automatic voltage stabilizer and the works pls send me the details to my email address.which is thanks.

  63. Ita E Bassey

    Am working on one of your circuit(street light) as a project.I need your help on written area(materials).Thanks

  64. where is vr-2, and the 555’s delay should be 10 seconds, not 10 milliSeconds!

  65. How can we make an automatic voltage stabilizer out of it ?

  66. hai sir iam a final year EEE student.i want to do a project on high voltage and low voltage cutoff with time delay(a model based on the circuit).you please give me the information regarding to the operation,you please send the information to my mail.

  67. Eng. Ahmed

    Thanks for this good cct.
    it’s good for our inst. that work in bad local Elec. Voltage

  68. twits4twats

    Mohamed Anany
    I am pretty confident that D4 and D5 should be 400 milli Watts and not 400 Watts. 400mW is suggested as it is a commonly manufactured value and therefore normally cheaper.

    • that was an error in the circuit diagram
      D4 and D5 are 400 milli watts. I have corrected the circuit diagram.
      Have a look.

      thanks to Mohammed and twits4twats for pointing out.

  69. Mohamed Anany

    Well thank you alot for your response, and i have another question, can i use a lower power diod than the (400w 5.6v D4 & D5 & D10)?, if you have an answer please post it or send me an email, again thank you for your fast response.
    best regards

  70. Mohamed Anany

    First it is a good circuit, but i want to ask about a replacement for the (SL100) transistor, because i have searched alot for that transistor but i didn’t find it. Please if you have an answer for this send me an email.
    best regards

    • you can try BD 139, BC 107 or simply any medium power npn transistor

  71. shubhrodutta

    I found your circuit simple and interesting.
    could you suggest the necessary change I need to do if I want to protect a 6V, 4.5 AH Lead Acid battery from overcharging and over discharging.
    To get a steady charging voltage of 7.3V for charging the battery, I will be using a 12V, 500 milliAmp Adapter, Send it through IC 7808 followed by diode 1N7004 in series. That will give me 7.3V.
    Thereafter I need a overcharge protection circuit.
    Similarly I need to protect the battery from over discharging.

    only 30% discharging will be good for me. it will give me a good life.
    will you be able to guide me.
    kind regards